Pumping systems wherein the pump is driven by a drive shaft or drive string are subject to torsional stresses which increase with the length of the shaft or string and are stored as reactive torque in the drive train. Many pumping systems also store a head of fluid in the production tubing which exerts further torsional stress on the drive shaft. When drive power to the system is interrupted, the reactive torque is released as backspin and, if an uncontrolled release of torque occurs, personal injury and/or property damage can result. This is the case, for example, in deep well down hole rotary pumps such as progressing cavity pumps. Rotary down hole pumps have been used in water wells for many years. More recently, especially progressing cavity pumps have been found well suited for the pumping of viscous or thick fluids such as crude oil laden with sand. Rotary down hole pumps are generally driven by sucker rod drive strings which usually have a relatively small diameter of 3/4 to 11/8 inches. Such drive strings are commonly used in wells that vary from 1,500' to 6,000' in depth, 3,000' being a common average. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to a bottom end of the drive string. The elongated drive string is subject to considerable torsional force which increases with the viscosity of the liquid being pumped and the displacement of the pump. This torsional force is stored in the elongated drive string as reactive torque. Several hundreds of revolutions of torsion can be stored in the drive string with a high-capacity pump or if viscous, sand laden crude oil is being pumped. When power is interrupted to the drive string, the reactive torque is released. Unless the release of reactive torque is controlled, violent backspin of the drive string will result, especially if an electric motor is used as a power source, which when disconnected from the power supply, offers almost no resistance to reverse rotation. This can lead to costly and undesirable damage to equipment and/or/personal injury to workmen in the vicinity of the equipment.
Various braking systems have been developed which provide for a controlled release of the reactive torque stored in the drive string of down hole rotary pumps upon interruption of drive power to the drive string. These are fluid brakes or hydraulically operated brakes, such as disclosed in commonly owned U.S. Pat. No. 5,358,036, or other braking systems which operate on centrifugal braking principles (U.S. Pat. Nos. 4,216,848 to Toyohisa Shiomdaira; 4,797,075 to Wallace L. Edwards et al; and 4,993,276 to Wallace L. Edwards). Although these braking systems are all intended to control the release of reactive torque stored in the drive train, they are all subject to possible failure due to wear, exposure to the elements, accidental damage etc. If power to the drive string is interrupted and the braking system fails, uncontrolled back-spin of the drive string will occur which can result in damage not only to the drive train but also to the braking system. Even more importantly, maintenance personnel unaware of the braking system failure and relying on the system releasing any reactive torque stored in the drive string after shut-off, can be severely injured. The violent, uncontrolled back-spin of the drive string observed in the absence of braking systems or with improper or damaged braking systems has led to drive pulleys exploding, sticking up drive string ends breaking off, and electric drive motors coming apart. Thus, a back-up system is desired which would prevent damage to the equipment rotating the drive string and injury to personnel upon back-spin of pump drive shafts caused by the release of reactive torque stored therein.